The measurement of the earth's magnetic field in several places of the world for centuries has shown that it varies with time. The second difficulty is that the radio active decay does not take place at a uniform rate but is a random process, and is therefore, governed by the laws of statistical probability. The polarity is recorded by the orientation of magnetic crystals in specific kinds of rock, and researchers have established a timeline of normal and reversed periods of polarity.
The best results can be obtained from specimens, which were preserved under very dry conditions, or even enclosed in rock tombs of the like. Electronic spin resonance: ESR, which measures trapped electrons using magnetic fields, is related to magnetic resonance imaging, the medical technique that allows doctors to look for tumors or peek inside your creaking knee. These changes obviously effect the flora and fauna, which try to adapt to the condtions by subjecting some changes in the body structure. Chemical Dating Techniques Although not that widely used, archaeologists do have a number of chemically based dating methods to choose from. Historical archeologists have found increasingly detailed ways to partition time on their sites.